you might be getting a lovely hardware conflict/swap.
so do you have one onboard sound card and one pci one?
if you are make sure that the onboard one is disabled in your bios.
this still may not stop linux using it but could do the trick.
the other thing to test is if you remove you pci sound card do you get audio all the time from the on board one?
what could be happening is that you installed mint and it detected you pci sound card and set it up. but due to crazy hardware/bios gremlins some times the on board sound is found and used, but the config is setup for the pci one so no sound.